Relaxation effect of chloramphenicol on the stringent control in Escherichia coli

In 10B601 (rel+) strain possessing a temperature-sensitive valyl-tRNA synthetase, chloramphenicol prevented the formation of guanosine-3'-diphosphate-5'-diphosphate (ppGpp) as well as the stringent control of stable RNA synthesis, under the conditions where the incorporation of valine into protein was still detectable i.e. at the lower restrictive temperatures. On the other hand, the effect of chloramphenicol was not observed at higher restrictive temperatures above 42 degrees C where the incorporation of valine was completely absent. Pretreatment of 10B601 cells with chloramphenicol before transfer to a high restrictive temperature (43.5 degrees C) did retard the onset of accumulation of ppGpp after the shift-up. Duration of the lag period was dependent on the concentration of chloramphenicol added. In parallel with the inability of the cells to accumulate ppGpp, stable RNA synthesis was permitted to continue at that high temperature. These results suggest that chloramphenicol traps aminoacyl-tRNA at the A-sites of ribosomes by damming-up the small flow of aminoacyl-tRNA under the restrictive supply of amino acids. Unchanged tRNA which has been located at the A-site is replaced by the charged one, thus resulting in the suppression of ppGpp formation and in the restoration of stable RNA synthesis.